Graphene field-effect transistors and devices for advanced high-frequency applications
Doktorsavhandling, 2019

New device technologies and materials are continuously investigated, in order to increase the bandwidth of high-speed electronics, thereby extending data rate and range of applications. The 2D-material graphene, with its intrinsically extremely high charge carrier velocity, is considered as a promising new channel material for advanced high frequency field-effect transistors. However, most fabrication processes introduce impurities and defects at the interface between graphene and adjacent materials, which degrade the device performance. In addition, at high drain fields, required for high transistor gain, the close proximity of the adjacent materials limits the saturation velocity, and there is a significant increase in the channel temperature caused by self-heating.

In this thesis, the influence of impurities and defects on charge transport, the limitations of the saturation velocity, and the effect of velocity saturation and self-heating on the transit frequency (fT) and the maximum frequency of oscillation (fmax) of graphene field effect transistor (GFETs) are analysed.

In addition, GFETs with state-of-the-art extrinsic fT =34 GHz and fmax =37 GHz, and an integrated 200-GHz GFET based receiver are presented. Also, through the development of a fabrication process of GFETs with a buried gate configuration, this work contributed to the direct nanoscopic observation of plasma waves in the GFET channel during terahertz illumination.

The study was conducted by (i) setting up a model describing the influence of impurities and defects on capacitance and transfer characteristics at low electric fields, (ii) by developing a method for studying the limiting mechanisms of the charge carrier velocity in the graphene channel at high electric fields and answering the question whether velocity saturation improves fmax, (iii) by developing a method to study the channel temperature and its effect on fT and fmax.

It was found that scattering by remote optical phonons limits the saturation velocity and charge carriers emitted from interface states at high fields are preventing the current to saturate and, hence, limiting fT and fmax. Additionally, the study shows that the channel temperature in GFETs can increase significantly causing degradation of the high frequency performance due to the decrease of charge carrier mobility and velocity.

In summary, this work shows that it is necessary to develop a GFET design and fabrication process providing clean and defect-free interfaces, to minimise parasitic effects, and to use materials with higher optical phonon energies and higher thermal conductivities than those used today. This will allow for realisation of GFETs with extrinsic fT and fmax in the sub-terahertz range.

field-effect transistors

microwave devices

graphene

carrier transport

impurities and defects

remote phonons

saturation velocity

traps

self-heating

Kollektorn, MC2, Kemivägen 9, Gothenburg
Opponent: Prof. Claire Berger, Georgia Institute of Technology, Atlanta, USA

Författare

Marlene Bonmann

Chalmers, Mikroteknologi och nanovetenskap (MC2), Terahertz- och millimetervågsteknik

Graphene field-effect transistors with high extrinsic fT and fmax

IEEE Electron Device Letters,; Vol. 40(2019)p. 131-134

Artikel i vetenskaplig tidskrift

Charge carrier velocity in graphene field-effect transistors

Applied Physics Letters,; Vol. 111(2017)p. 233505-

Artikel i vetenskaplig tidskrift

Effect of oxide traps on channel transport characteristics in graphene field effect transistors

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures,; Vol. 35(2017)p. 01A115-

Artikel i vetenskaplig tidskrift

Effects of Self-Heating on fT and fmax Performance of Graphene Field-Effect Transistors, Accepted in IEEE Transactions on Electron Devices 2020

För att öka bandbredden för höghastighetselektronik, och därigenom utvidga datahastigheten och tillämpningsområden, undersöks kontinuerligt nya teknologier och material. Väsentliga komponenter i dessa elektroniska enheter är transistorer. 2D-materialet grafen, med sin extremt höga laddningshastighet, betraktas som ett lovande nytt kanalmaterial för avancerade högfrekventa fälteffekttransistorer. Hittills har de flesta tillverkningsprocesser oavsiktligt infört föroreningar och defekter vid gränssnittet mellan grafen och angränsande material, vilket försämrar enhetens prestanda. I denna avhandling tillverkas grafen-fälteffekt transistorer (GFET) och de begränsande faktorerna analyseras. Detta möjliggör en beskrivning av vägar mot förverkligandet av GFETer med högfrekvensprestanda.

New device technologies and materials are continuously explored, in order to increase the bandwidth of high speed electronics, thereby extending the data rate and range of applications. Essential components in those electronic devices are transistors. The 2D material graphene, with its extremely high charge carrier velocity, is considered as a promising new channel material for advanced high frequency field-effect transistors. However, so far, most fabrication processes unintentionally introduce impurities and defects at the interface between graphene and adjacent materials, which degrade the device performance. In this thesis, graphene-field effect transistors (GFETs) are fabricated and the limiting factors are analysed. This allows for outlining paths towards the realisation of GFETs with high-frequency performance.

Styrkeområden

Informations- och kommunikationsteknik

Nanovetenskap och nanoteknik (2010-2017)

Infrastruktur

Kollberglaboratoriet

Nanotekniklaboratoriet

Ämneskategorier

Nanoteknik

ISBN

978-91-7905-237-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4704

Utgivare

Chalmers tekniska högskola

Kollektorn, MC2, Kemivägen 9, Gothenburg

Opponent: Prof. Claire Berger, Georgia Institute of Technology, Atlanta, USA

Mer information

Senast uppdaterat

2020-01-09